The encoding method for contemporary input apparatus, for example, the keyboard for personal computer (PC), are using the two-dimensional keyboard scan process to define the function of individual keys. As an example of a keyboard encoder IC, commonly it is available as a 40-pin DIP package. The pin assignment of the 40-pin packaged keyboard encoder IC chip is arranged in such a manner that, 24 pins are assigned to define the key functions of a prevalent keyboard for PC with 16×8 keys, 2 oscillating pins, 3 LEDs and clock & data lines. As a result, 33 pins are necessary to complete the keyboard scan process.
The conventional key structure of a keyboard is illustrated in FIG. 1(a). When key 11 is pressed, the first electrode 12 will come into electrical engagement with the second electrode 13. The electrical engagement between the first electrode 12 and the second electrode 13 can be best understood in reference to FIG. 1(b). In FIG. 1(b), the X-axis electrode 121 on the lower thin film and the Y-axis electrode 131 on the upper thin film are interlaced to form a two-dimensional matrix, in which each of the coordinate address (X-coordinate, Y-coordinate) represents a key function, as shown in the look-up table of FIG. 1(c). As the key is pressed to enable the X-axis electrodes and the Y-axis electrodes to come into an electrical engagement, the keyboard will detect the coordinate address (X-coordinate, Y-coordinate) of the intersection of the X-axis coordinate and the Y-axis coordinate, and further the key for which is pressed can be known.
The keyboard scan process for identifying which key is pressed is illustrated in FIG. 1(d). The keyboard encoder will output a low-level signal through the outputs of each of the Y-axis electrodes, and detect whether or not any of the inputs of the X-axis that is originally pulled to a high level is changed. Accordingly, when a key is pressed, a certain input of the X-axis electrode will transit from high level to low level, whereby identifying the key being pressed.
In addition to the identification of the key being pressed, the disposition of the phantom key is another important task. In order to avoid the occurrence of the phantom key, the art of properly arranging the composite keys, such as Ctrl key or Shift key that are likely to be pressed is an effective way to address these problems.
However, how to lower the cost without affecting the performance of the keyboard is a knot that all the keyboard manufacturers are seeking ways to break through. It would be highly desirable to provide an input apparatus that makes use of a different encoding method to perform keyboard scan process, and thus a keyboard encoder IC with less pins, reduced manufacturing cost and enhanced reliability can be obtained.